Is it possible to do a Rigid Body chain that needs to split at a certain frame?
Yes - you can animate the properties of the rigid body constraints to effectively disable the link.

For example, here is a chain of cubes, linked together with a series of Rigid Body Constraints - each set to a Stiffness of 40. At a certain frame the keyframes Stiffness drops to zero, splitting the 'chain' :



The produces the following when animated :